1. Field of the Invention
The present invention relates to a method of anodic bonding and, in particular, to a method of bonding a conductor or a semiconductor wafer to an inorganic insulating material by an anodic bonding method.
2. Description of the Related Art
FIG. 1 is a cross sectional view schematically illustrating a semiconductor wafer and an inorganic insulating material being bonded by a conventional anodic bonding method. In the figure, a semiconductor wafer 1 with integrated circuits formed its surface and an inorganic insulating material 2, such as Pyrex glass, placed on the semiconductor wafer 1, are placed above a heating device 3 having a heater for heating these materials. A cathode 5 connected to a cathode terminal 4 is pressed against the inorganic insulating material 2. The semiconductor wafer 1 acts as an anode, to which an anode terminal 6 is electrically connected. The semiconductor wafer 1, the inorganic insulating material 2, bonded by a conventional anodic bonding method, are constructed as mentioned above and the bonding of these materials is performed as described below. First, the semiconductor wafer 1 and the inorganic insulating material 2 placed above the heating device 3 are heated by the heating device 3 to between 200.degree. and 400.degree. C. Next, a DC voltage of 300 to 800 V is applied between the anode terminal 6 and the cathode terminal 4. The application of the DC voltage causes ions in the inorganic insulating material 2 to move. The close contact between the wafer 1 and insulating material 2 caused by an electrostatic attracting force spreads concentrically directly under and with the cathode 5 as a center, and an oxide film is formed in the interface between the semiconductor wafer 1 and the inorganic insulating material 2, as a result of the diffusion of oxygen, and is bonded.
An anodic bonding method mentioned above is often performed in a vacuum. Because heat is dissipated in a vacuum, a difference in temperature of approximately 20.degree. to 50.degree. C. occurs between the heating temperature of a heater or the like and a bonding interface and the surface of the inorganic insulating material 2 (which in this case refers to the surface on which the cathode 5 is disposed). For this reason, a thermal stress occurs within the inorganic insulating material 2, and in order to increase the temperature of the entire inorganic insulating material 2 to a temperature at which the ions within the inorganic insulating material 2 can move, the temperature of the heating device 3 must be set higher. Hence, warping or residual stress may occur in the semiconductor wafer 1 and the inorganic insulating material 2. Since electric fields are concentrated in the cathode 5, the residual stress is larger in the vicinity of the cathode 5. Thus, in attempting to minimize the size of the area in which the residual stress is received after bonding by making the area of the cathode 5 smaller, as shown in FIG. 2, the cathode 5 is sometimes disposed at the outer periphery of the semiconductor wafer 1. Even in this case, however, there occurs a problem in that there is a stress distribution with the position of the cathode 5 as a peak, so that a uniform bonding cannot be performed and a bonding speed is slow. Hence, to up the bonding speed, as shown in FIG. 3, the area of a cathode 7 is made larger. In this case, the cathode 7 is an electrode having a thickness of several mm, for example, of stainless steel and it is removed from the inorganic insulating material 2 after bonding is terminated. However, fine gaps may occur between the cathode 7 and the inorganic insulating material 2. In such a state, a voltage will not be applied uniformly. Problems arise in that voids occur between the semiconductor wafer 1 and the inorganic insulating material 2 which have been bonded, or the bonding becomes non-uniform, and further the amount of warping becomes larger.